Electromagnetic radiation has long been employed for the remote detection and identification of targets. In particular, electromagnetic radiation in the form of radar has been employed in airborne platforms for the detection and classification of targets on the ground. Synthetic aperture techniques have been employed with such airborne radar systems in order to improve the imaging capability of the relatively small antenna systems which can be mounted on moving platforms. This technique involves continuous active transmission of electromagnetic radiation and reception of returns from a target area of interest, while the platform is in motion relative to the area of interest. The motion of the platform enables the formation of a synthetic aperture which is much larger than the aperture of any actual antenna which can be carried on the moving platform. This technique thus increased the imaging capability of the sensing system on the moving platform.
Such active imaging systems have a disadvantage in a hostile combat environment. These systems must announce the presence of the moving platform by the radar transmissions necessary to form the image. The imaging system requires the detection of a returned radar echo which must travel twice the range between the moving vehicle and the area of interest. On the other hand, a receiver located within the area of interest need only detect the transmissions having come directly from the moving vehicle. Therefore, the target in the area of interest always has the advantage in detection of the moving vehicle over the chances of the synthetic aperture radar on the moving vehicle detecting the target and will generally be able to detect the sensing system on the moving platform prior to its detection by the sensing system on the moving platform. The target may be able to take evasive action to hide itself or initiate counter measures against the moving platform prior to the imaging system on the moving platform being able to detect the presence of the target. This fact means that a passive imaging system, that is a system which does not provide its own transmissions of radiation, would be highly advantageous in this environment.